1. Field of the Invention
The present invention relates to a communication channel assigning system corresponding to DS (Direct Sequence)-CDMA (Code Division Multiple Access) radio modulation method and FH (Frequency Hopping)-CDMA radio modulation method for use with a mobile communication system such as a portable telephone system.
2. Description of the Related Art
In recent years, in mobile communication systems, as well as audio information, data communication system with a higher bit rate than audio information has been required for communications. As a modulation system that satisfies such a requirement, a CDMA radio communication modulation system is known.
In the conventional CDMA radio modulation system, each of various frequency bands (for example, a 1-MHz band as a narrow band, a 5-MHz band as a intermediate band, and a 20-MHz band as a wide band) is assigned a predetermined number of communication channels. The number of communication channels that can be used at a time in each band (namely, the interference power amount of each band) is fixed to a predetermined value.
Thus, when the number of calls that take place in one frequency band exceeds a predetermined value, even if another frequency band has free channels, the excessive calls cannot be assigned to the free channels. Thus, in the conventional mobile communication system, frequency bands cannot be sufficiently used. In other words, the conventional mobile communication system has a large division loss.